Method of forming a protective bi-layer coating on phosphore particles

ABSTRACT

The method of making a bi-layer coating on phosphor particles is disclosed. The first layer surrounding the phosphor is silica. The second layer surrounding the phosphor is alumina. The bi-layer phosphor is useful in fluorescent lamps providing improved maintenance and brightness. The bi-layer phosphor can also be used in high color rendition lamps employing blends of phosphors.

The present invention relates to improved manganese activated zincsilicate phosphors. More specifically, the present invention involvedcoating a willemite phosphor with a layer of silica and then a coatingof alumina. The "bi-layer" phosphor obtained shows improved lumencharacteristics and is particularly useful in fluorescent lamps.

BACKGROUND OF THE INVENTION

During certain phosphor synthesis and lamp fabrication steps, the finelydivided luminescent materials may be exposed to oxidizing (oxygen-rich)atmospheres at elevated temperatures. An example of this is theso-called `lehring` process used to burn away organic aqueous lampcoating dispersion. It is well known that the brightness of the finishedfluorescent lamp may be reduced significantly as a result of the lehringoperation (the so-called `lehrloss`). This reduction in brightness mayresult from a partial oxidation of reactive low valence ions present inthe phosphor lattice.

A somewhat more involved example relates to the process in U.S. Pat. No.4,585,673 wherein the formation of protective coatings (typicallyalumina coatings) upon the surfaces of finely divided phosphor particlesvia chemical vapor deposition using an organometallic precursor in agas-fluidized bed is disclosed. When manganese-doped zinc silicate isalumina-coated via the process described in the '673 patent, and whenfluorescent lamps are fabricated from the coated phosphor producedtherefrom, these lamps display much better lumen maintenance than dosimilar lamps fabricated using the virgin (uncoated) zinc silicatephosphor. During the fabrication of such lamps, the phosphor particlesare typically dispersed in an aqueous medium. Unfortunately, if thewater-based suspension is held-over for several days before use (atypical situation), the beneficial effects associated with the '673coating are lost.

This `holdover` problem can be overcome however, by annealing thealumina-coated phosphor in the air at a temperature between about 700°C. and about 850° C. for a period of time ranging from about 15 minutesto about 20 hours as described in U.S. Pat. No. 4,805,400.Unfortunately, while this coated phosphor annealing process solves theholdover problem, it also causes the zinc silicate phosphor to reactwith the alumina coating. Zinc and manganese diffuse into the aluminacoating, probably forming a mixture of zinc and manganese aluminates.The coated phosphor develops a `body color`, and suffers a reduction invisible light emission upon exposure to an ultraviolet light source.Moreover, very similar phenomena are observed, as well, when the virgin(uncoated) phosphor is subjected to the annealing process. The increasedbody color and reduced brightness which result from annealing both theuncoated and the '673 coated zinc silicate phosphor are believed topartially result from the oxidation of some of the divalent manganeseions located on the surface of the uncoated phosphor particles or withinand on the surface of the reactive alumina coating.

Prior to the present invention, there was no known means of preventingthese detrimental interactions between the phosphor and the oxygen-richatmosphere within the annealing furnace. By means of the methoddescribed below, these detrimental interactions are virtuallyeliminated, allowing a phosphor coated by the process disclosed in the'673 patent to be thoroughly annealed without suffering reflectance orbrightness losses.

Another aspect of the present invention involves the use of highbrightness zinc silicate phosphors as components of the triblendphosphors. As discussed previously, coated zinc silicate phosphors areunstable in the water based suspension systems used to manufacturefluorescent lamps. The zinc silicate phosphors must be annealed tostabilize the coated phosphor. However, the performance of a zincsilicate phosphor suffers both in terms of brightness output and lumenmaintenance after annealing. Attempts to improve the base phosphorperformance prior to annealing include remilling and refiring (RMF) thephosphor. Such an alumina coated "RMF" phosphor shows improved lumencharacteristics when used as a component in the high color renditiontriblend layer. However, the remilling and refiring process results in alarge loss of starting material, thus increasing cost of the phosphor.The present invention solves these problems in a novel and economicalway.

In a related application, an improved compact fluorescent lamp can bemanufactured using a coated willemite phosphor as the green-emittingcomponent. Compact fluorescent lamps of the twin-tube and doubletwin-tube variety have become important for energy conservation inrecent years since they have efficiencies which far exceed those ofconventional incandescent lamps. While these lamps are verycost-effective with very short payback periods, they, nevertheless, havehigh initial costs which have limited the scope of applications in whichthey have been exploited. Therefore, it is desirable to further reducethe cost of these lamps through the use of less expensivenon-rare-earth-containing substitutes.

The compact fluorescent lamps currently employ two rare-earth basedphosphors. They are Y₂ O₃ :Eu (Sylvania Type 2342) for the red emissionand Ce,Tb Mg Aluminate: Ce, Tb (Sylvania Type 2293) for the greenemission. No blue-emitting phosphor is required, since the bluecomponents of the mercury discharge are used to achieve the proper colortemperature of the emitted `white` light. More recently, LaP0₄ :Ce,Tb,manufactured by Nichia Corporation, is being considered as a replacementfor the Type 2293. Because these materials contain expensive rare-earthsas the activators, they are some of the most expensive phosphorscommercially used.

As mentioned previously, a green-emitting zinc orthosilicate phosphoractivated with manganese, also known by the mineral name willemite canbe improved by the application of a bi-layer coating prior to annealing.The bi-layer consists of a thin coating of silica applied between thebase phosphor and a conformal alumina coating which is exposed to themercury discharge. The base phosphor is a zinc silicate phosphor dopedwith manganese and tungsten as, described in our commonly assigned U.S.patent application (06/902,265), now abandoned. This phosphor can bemanufacture on production scale equipment using a single step firingprocedure which provide very high yields (typically 90%). These highyield and efficiencies of scale provide substantial phosphor costsavings which far outweigh the cost of applying the intermediate silicalayer.

Finally the use of the willemite phosphor which has been coated withsilica and then with alumina can be used as the green-emitting componentof high color rendition fluorescent lamp.

SUMMARY OF THE INVENTION

A method for forming a continuous layer of silica on phosphor particlesis disclosed. The method comprises vaporizing a silicon containingprecursor such as tetramethyloxysilane (TMOS) or tetraethoxyorthosilane(TEOS) into an inert carrier gas and passing this gas containing TMOS orTEOS through a phosphor powder wherein the phosphor particles areenveloped in the TMOS or TEOS at a temperature of greater than 400° C.An oxidizing gas is passed into the phosphor powder which reacts withthe TMOS to form a continuous coating of silica on the phosphorparticles. The resulting silica coated phosphor can then be furthercoated with alumina.

In another aspect of the present invention, fluorescent lamps using thedouble coated bi-layer phosphor are manufactured. The resulting lampsshow improved lumen and maintenance performance.

In yet another aspect of the present invention the double coatedphosphor can be used in high color rendition fluorescent lamps. Thebi-layer phosphor replaces the green emitting rare earth phosphors thatare presently used in these lamps. The bi-layer phosphor results in aless expensive lamp as the expensive green-emitting phosphor arereplaced while the lamps suffers almost no loss in lumen output.

OBJECTS OF THE INVENTION

It is an object of the present invention to improve the brightness of afluorescent lamp which contains a willemite phosphor by protecting thephosphor with an alumina/silica bi-layer.

Yet another object of the present invention is to achieve substantialbrightness improvements in both large and small particle size willemitephosphor.

A further object of the present invention is to provide a silicadiffusion barrier over the surface of a phosphor to prevent detrimentalreactions from occurring during the annealing of the phosphor.

Yet another object of the present invention is to provide a compactfluorescent lamp of the twin-tube variety which uses a two-layer coatedwillemite phosphor as a component in the phosphor blend.

Another object of the present invention is to provide a two-layer coatedwillemite phosphor as the green-emitting component of single andmultiple layer high color rendition fluorescent lamps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an apparatus suitable forcoating phosphor particles.

FIG. 2 is a bed temperature versus time graph for a TMOS/O₂ coating runusing a manganese activated zinc silicate phosphor.

FIG. 3 shows the weight percent of silica on phosphor powder as afunction of coating time.

FIG. 4 shows the relative plaque brightness versus weight percent ofsilica coating on a zinc silicate phosphor.

FIG. 5 shows a cross-sectional view of a phosphor particle coated with alayer of silica which is coated with a layer of alumina.

FIG. 6 shows an elevational view of double coated fluorescent lamp.

FIG. 7 shows a cross-sectional view of the lamp of FIG. 6.

FIG. 8 shows the color points taken at 3 positions of a double coatedlamp using Type 2293 phosphor.

FIG. 9 shows the color points taken at 3 positions of double coated lampusing an "RMF" phosphor.

FIG. 10 shows the color points taken at positions of a double coatedlamp using the phosphor of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

One aspect of the present invention involves the formation of acontinuous and conformal coating of silica on the surfaces of zincsilicate or cool white phosphor particles via chemical vapor deposition(CVD) while the phosphor particles are suspended within an isothermalgas fluidized bed. In a second aspect of the present invention silicacoatings are used to prevent reductions in brightness and thedevelopment of body color when manganese activated zinc silicate (Zn ₂SiO₄ :Mn) phosphors are heated in air at temperatures above about 600°C. These silica coatings also act as diffusion barriers, preventing themigration of zinc and manganese from the surface of the Zn₂ SiO₄ :Mnphosphor through the silica coating and therefore also throughcontinuous and conformal alumina coatings that may be formed on thesurfaces of the silica coated phosphor particles.

A schematic representation of the fluidized bed reactor used to coat thephosphor particles with silica is shown in FIG. 1. In FIG. 1, a feederline 11 carries the inert bubbler gas through valve 54 into a stainlessbubbler 12 which contains a silicon containing precursor such astetramethoxysilane (TMOS) or tetraethoxyorthosilane (TEOS). In thebubbler12, the coating precursor, TMOS or TEOS is vaporized into thebubbler gas. The bubbler is heated by heating means such as heating tape(not shown). The bubbler gas containing the TMOS or TEOS can be dilutedby carrier gas to provide appropriate concentration of reactants. Thebubbler gas containing the vaporized TMOS or TEOS is carried throughconnector line 13and is diluted by the carrier gas at valve 55 which iscarried through line111. Lines 13 and 111 join and the resulting line isheated by heating tape30 or other means. The bubbler and carrier gaswith the TMOS passes througha stainless steel plenum 40 which ismaintained at a temperature of about 32° C. The carrier gas along withthe vaporized TMOS or TEOS then flows through a porous stainless steelgas distributor 14. The gas then flows into a quartz glass reaction tube15. Within the reaction tube 15 isa vibrating mixer 17.Circumferentially located on the shaft of the vibrating mixer 17 andnear the vibrating disc 19 are a series of holes 18through which theoxidizing gas with or without an inert diluting gas enters the reactiontube 15. Oxygen is introduced to the reaction tube through line 21. Nodiluting gas means for the oxygen is shown in FIG. 1. The quartz glassreaction tube is surrounded by a furnace 20.

EXAMPLE 1

Aluminum Oxide C (0.1%) was blended with each phosphor as a fluidizationaid. The temperature of the fluidized bed reactor was maintained between450° C. and 460° C. during the coating process. Further, dueto themoisture produced within the high temperature fluidized bed as abyproduct of the TMOS oxidation reaction, the fluidized bed remainedalmost perfectly isothermal from the beginning to the end of eachcoating process run. A typical bed temperature versus time curve for aTMOS/O₂ coating run is shown in FIG. 2. In a typical run, 400 gm of thephosphor are coated using a 32° C. bubbler temperature, with 0.5 l/minnitrogen gas (the fluidizing gas medium) flowing through the TMOSbubbler, and with 0.6 l/min oxygen gas entering the fluidized powder bed(through the hollow stirrer rod) at a point a few centimeters above thelevel of the porous distributor plate.

Coating reactions were carried out for times ranging between 2.5 hoursand 7.5 hours. Subsequently, the amounts of silica deposited weredetermined analytically for several of the coated phosphors. The resultsof these determinations are listed in Table 1. These data are plotted vscoating time in FIG. 3. As shown, the amount of silica deposited via theTMOS/O₂ coating reaction increases linearly with increasing coatingtime.

                  TABLE 1                                                         ______________________________________                                        Results of Chemical Analyses for Silica                                       Coatings on TMOS/O.sub.2 -Coated Zn.sub.2 SiO.sub.4                           and Cool White Phosphors*                                                     Phosphor   Coating Time (hr)                                                                          Weight Percent SiO.sub.2                              ______________________________________                                        Zn.sub.2 SiO.sub.4                                                                       5            1.65                                                  Cool White  21/2        0.80                                                  Cool White 5            1.77                                                  Cool White  71/2        2.56                                                  ______________________________________                                        *400 gm phosphors 0.5 l/min bubbler flow rate; 32° C. bubbler           temperature; 0.6 l/min O.sub.2 flow rate; inert carrier gas for TMOS:         N.sub.2.                                                                 

The silica-coated and uncoated zinc silicate and cool white phosphorswere also examined via high resolution scanning electron microscopy.Photographic images were obtained at 20,000× and 50,000× magnification.There were no features observed in the photomicrographs obtained withthe silica-coated materials that were not observed in thephotomicrographs obtained with the corresponding uncoated phosphors.Thus,the silica coatings produced via the TMOS/O₂ reaction appear to beuniform and conformal to the surfaces of the underlying phosphorparticles.

The continuity of the silica coatings formed on the ZnSiO₄ :Mn andcool-white phosphors were examined using X-ray photoelectronspectrometry.Typical normalized relative atomic concentration dataobtained with an uncoated and a silica-coated ZnSiO₄ :Mn phosphor arecompared in Table 2. Typical data obtained with an uncoated and asilica-coated cool white phosphor are similarly compared in Table 3. Asshown in Table 2, signals corresponding to zinc and manganese arecompletely absent from theXPS spectra obtained with the TMOS/O₂ -coatedzinc silicate phosphor. Similarly, except for a very small calciumsignal, the XPS spectra obtained with the TMOSO₂ -coated cool whitephosphor contain no evidence of the underlying phosphor. Thus, thesilica coatings formed uponthe surfaces of the phosphor particles appearto be continuous as well as conformal.

                  TABLE 2                                                         ______________________________________                                        Relative Atomic Concentrations of Surface                                     Elements from XPS Analyses of Uncoated and                                    Silica-Coated Zn.sub.2 SiO.sub.4 :Mn                                          Coating        Zn(3p)    Si(2p)  Mn(2p)                                       ______________________________________                                        None           100        73     2                                            ≃2 w/o SiO.sub.2                                                                0        100     0                                            (from TMOS/O.sub.2                                                            reaction)                                                                     ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Relative Atomic Concentrations of Surface                                     Elements from XPS Analyses of Uncoated and                                    Silica-Coated Cool White Phosphor                                             Coating        Ca(2p)    F(1s)   P(2s)                                        ______________________________________                                        None           100       23      63                                           ≃2 w/o SiO.sub.2                                                               <1         0       0                                           (from TMOS/O.sub.2                                                            reaction)                                                                     ______________________________________                                    

EXAMPLE 2

Three different lots of Zn₂ SiO₄ :Mn phosphor were coated with silicaaccording to the method described in Example 1. Samples of eachphosphor, before and after silica-coating, were annealed in air for 4hours at 750° C. Additional samples were likewise annealed in the airfor one hour at 800° C. Portions of each annealed material werepressedinto so-called plaques (i.e., they were pressed into molds so thatuniformly flat horizontal surfaces were obtained.) A spot brightnessmeterequipped with a green photo-optic filter along with an unfilteredmercury plasma ultraviolet light source was used to measure a so-calledplaque brightness for each sample, expressed relative to that of asample of eachuncoated and unannealed phosphor. The results of thesemeasurements are listed in Table 4. The relative plaque brightnessesmeasured with phosphorlot #1 are also plotted vs w/o silica added duringthe TMOS/O₂ -coating process in FIG. 4.

                  TABLE 4                                                         ______________________________________                                        Relative Plaque Brightness of TMOS/O.sub.2 -Coated Zn.sub.2 SiO.sub.4         :Mn*                                                                          Phosphor w/o SiO.sub.2                                                                            Anneal     Relative Plaque                                Lot      Coating    Conditions Brightness (%)                                 ______________________________________                                        1        0          none       100.0                                                               4 hr/750° C.                                                                     95.6                                                               11/2 hr/800° C.                                                                   96.0                                           1        0.40       none       94.8                                                                4 hr/750° C.                                                                     99.3                                                               11/2 hr/800° C.                                                                   100.4                                          1        0.80       none       86.9                                                                4 hr/750° C.                                                                     99.3                                                               11/2 hr/800° C.                                                                   100.4                                          1        1.20       none       84.2                                                                4 hr/750° C.                                                                     99.3                                                               11/2 hr/800° C.                                                                   100.4                                          2        0          none       100.0                                                               4 hr/750° C.                                                                     95.9                                                               11/2 hr/800° C.                                                                   95.9                                           2        1.20       none       78.1                                                               11/2 hr/800° C.                                                                   100.3                                          3        0          none       100.0                                                               4 hr/750° C.                                                                     95.7                                                               11/2 hr/800° C.                                                                   95.9                                           3        1.20       none       80.0                                                               11/2 hr/800° C.                                                                   101.9                                          ______________________________________                                        *Bubbler Temp. = 32-33° C.; Bubbler Flow Rate = 0.5 1/min; O.sub.2      Flow Rate = 0.6 l/min; Powder Weight = 400 gm; Coating Temp =                 450-460° C.                                                       

As shown, the brightness of the uncoated phosphor is lowered by at least4%when annealed in the air at 750° C. or at 800° C. A body color(corresponding to a reduction in reflected visible light) also developsduring the annealing of the uncoated phosphor. On the other hand,sizablereductions in brightness are also observed with the unannealedsilica-coated phosphors. Moreover, the thicker the silica coating, thelower the measured plaque brightness. For instance, the plaquebrightness measured with a zinc silicate phosphor coated with 1.20 w/osilica (via the TMOS/O₂ reaction) is only around 80% of that measuredwith the uncoated phosphor.

In contrast, plaque brightnesses nearly equal to or exceeding thosemeasured with the uncoated and unannealed phosphor are obtained withsilica-coated phosphors that have been annealed in the air attemperaturesbetween 750° C. and 800° C. Such silica-coated and annealedmaterials are also notable for an absence of the body-color thatdevelops during the annealing of the uncoated phosphor. Therefore,whereas the uncoated phosphor cannot be air-annealed without suffering a4%-5% reduction in plaque brightness as well as a reduction in reflectedvisiblelight, the brightness of a silica-coated phosphor actuallyincreases with increasing annealing temperature to a level exceedingthat measured with the uncoated and unannealed phosphor, itself.

EXAMPLE 3

The implication of the data shown in Example 2 is that detrimentalinteractions which normally occur between the phosphor and the airduring the annealing step are prevented when the phosphor is coated witha thin layer of silica. The body-color that develops during theannealing of the uncoated Zn₂ SiO₄ :Mn phosphor suggests that the Mn²⁺ions located on or near to the phosphor particle surface are oxidizedduring the anneal. However, it is possible that the body color is due tooxidizedtungsten at the phosphor particle surface. The absence of thisundesirable body-color, the undiminished brightnesses obtained withannealed silica-coated Zn₂ SiO₄ :Mn phosphors, and the observedcontinuity and conformality of the silica coatings themselves indicatethat the phosphor surface is stabilized by the presence of the coating,thereby preventing the surface manganese from interacting with theoxidizing atmosphere within the annealing furnace.

That the phosphor surface is stabilized by the presence of the silicacoating may also be shown by examining the coated phosphor using x-rayphotoelectron spectrometry. Listed in Table 5 are the normalizedrelative atomic concentrations of Al, Zn, Si, and Mn obtained withseveral samples from the measured XPS signal intensities correspondingto the Al(2p), Zn(3p), Si(2p), and Mn(2p) electrons, respectively.Sample 1 is a Zn₂SiO₄ :Mn phosphor coated with alumina according to themethod described in U.S. application 07/432,941, now U.S. Pat. No.4,999,219 using aluminum isopropoxide (AIP) as the organometalliccoating precursor.Sample 3 was obtained by coating the same phosphorwith silica as describedin Example 1 (using TMOS as the coatingprecursor). Samples 2 and 4 were obtained by air-annealing samples 1 and3, respectively, for 4 hours at 750° C.

                  TABLE 5                                                         ______________________________________                                        Relative Atomic Concentrations for XPS Analyses                               AIP/O.sub.2 -Coated, TMOS/O.sub.2 -Coated, and TMOS/O.sub.2 AIP/O.sub.2       Coated Zn.sub.2 SiO.sub.4 :Mn                                                 Sample                                                                        No.   Material       Al(2p)  Zn(3p)                                                                              Si(2p)                                                                              Mn(2p)                               ______________________________________                                        1     AIP/O.sub.2 -Coated .sup.a                                                                   100     0     0     0                                          Zn.sub.2 SiO.sub.4                                                      2     Sample 1, annealed                                                                           100     13    0     3                                          4 hr 750° C.                                                     3     TMOS/O.sub.2 -Coated .sup.b                                                                   0      0     100   0                                          Zn.sub.2 SiO.sub.4                                                      4     Sample 3, annealed                                                                            0      0     100   0                                          4 hr 750° C.                                                     5     Sample 4, AIP-O.sub.2 - .sup.a,b                                                             100     0     0     0                                          Coated                                                                  6     Sample 5, annealed                                                                           100     0     0     0                                          4 hr 750° C.                                                     ______________________________________                                         .sup.a ≃2% Al.sub.2 O.sub.3 Coating                             .sup.b ≃2% SiO.sub.2 Coating                               

As shown in Table 5, none of the cations present in the Zn₂ SiO₄ :Mnphosphor are detected in the XPS spectra obtained with thealumina-coated phosphor. This indicates that the AIP/O₂ coating iscontinuous and thick enough to filter any Zn(3p), Si(2p), or Mn(2p)electrons that might be emitted under x-ray bombardment. In contrast,relatively large Zn(3p) and Mn(2p) signals are detected after annealingthe alumina-coated phosphor for 4 hours at 750° C. (sample 2). Theseresults are interpreted to indicate that these cations are mobile enoughto migrate through the alumina coating during the anneal. In sharpcontrast are the XPS data obtained with the TMOS/O₂ -coated samples. Inthis case, silicon is the only cationic species detected either beforeor after the 4 hour 750° C. anneal, indicating that the zinc andmanganese ions present on the surface of the Zn₂ SiO₄ :Mn phosphor donot migrate through the silica coating during the anneal. The fact thatthe alumina-coated phosphor possesses a distinct body-color after theanneal (thought to be due to oxidized manganese), whereas the annealedsilica-coated phosphor does not possess such a body-color can beunderstood from these data.

Finally, consider the results obtained using the Zn₂ SiO₄ :Mn phosphorthat has been silica-coated via the TMOS/O₂ reaction and subsequentlyair-annealed for 4 hours at 750° C. (sample 4). A quantity of thissilica-coated and annealed phosphor was coated with alumina via theAIP/O₂ reaction as described in U.S. patent application Ser. No.07/432,941 now U.S. Pat. No. 4,999,219. As shown in Table 5 (sample 5),Al is the only cationic species detected via XPS analysis of thismaterial, indicating that the alumina coating is continuous anduniformly thick enough to prevent the detection of any Si(2p) electronsthat might be generated under x-ray bombardment. Most significant is thefact that an identical result is obtained after annealing thedouble-coated phosphor for 4 hours at 750° C. In contrast to the resultsobtained with sample 2 (in the absence of the silica diffusion barrier),the absence of XPS signals indicating the presence of Zn, Si, or Mn nearto the surface of the annealed double-coated material and the completeabsence of any detectable body-color indicates that the silica coatingprevents the interaction between the phosphor and the alumina coatingthat would otherwise occur.

This conclusion is reinforced by the relative plaque brightness datalistedin Table 6. Shown are the measured brightnesses (relative to thatof the uncoated and unannealed phosphor) of AIP/O₂ -coated Zn₂ SiO₄ :Mnbefore and after a 4 hour 750° C anneal, both with and without a TMOS/O₂(silica) diffusion barrier. As indicated in thetable, the reduction inbrightness observed with the unannealed phosphor inthe absence of thediffusion barrier is more than twice that obtained with thedouble-coated phosphor. More significantly, the reduction in brightnessobserved with the annealed alumina-coated phosphor in the absence of thediffusion barrier is an order of magnitude greater than that obtainedwhen the alumina coating was applied over the diffusion barrier. Thus,the plaque brightness measured with the double-coated and annealedphosphor was only about 1% below that measured with the virgin phosphor.

                  TABLE 6                                                         ______________________________________                                        Relative Plaque Brightnesses of AIP/O.sub.2 -Coated Zn.sub.2 Sio.sub.4        :Mn                                                                           with and without underlying SiO.sub.2 Diffusion Barrier .sup.1                w/o SiO.sub.2 .sup.2                                                                   Anneal     Plaque Brightness Relative .sup.3                         Coating  Conditions to that of Uncoated Phosphor                              ______________________________________                                        0        none       94.2%                                                              4 hr/750° C.                                                                      87.7%                                                     1.20     none       97.3%                                                              4 hr/750° C.                                                                      98.8%                                                     ______________________________________                                         .sup.1 Ca. 2 w/o Al.sub.2 O.sub.3 Coating                                     .sup.2 SiO.sub.2 -coated phosphor was annealed 4 hours at 750° C.      prior to coating with alumina                                                 .sup.3 Plaque brightnesses relative to that measured with the uncoated        phosphor.                                                                

Due to the results obtained from Examples 1-3 it was thought that theresults of improved plaque brightness and lumen maintenance could beextended to fluorescent lamps. However, it is known that correlationbetween handlamp plaque brightness and fluorescent lamp performance fora given phosphor frequently do not exist. This results from a multitudeof factors including changes in the phosphor which occur during lampbaking, lamp fabrication and the contact of the phosphor with themercury discharge. Moreover, the mercury discharge within a fluorescentlamp contains about 15% of its emission at 185 nm. This short-wavelengthemission can lead to enhanced brightness and/or damage to the phosphorwhich can influence the observed initial brightness and maintenance.

More specifically, in the case of the alumina coated and annealedwillemitephosphor, the fluorescent lamps which have the highest lumenperformance possess a plaque brightness of 92% of the virgin phosphor.However, the corresponding fluorescent lamp performance may only be afew percent lower. Further, while the handlamp photoluminescentperformance is improved with a silica coated and annealed phosphor, thismaterial exhibits a significant loss in brightness and catastrophicmaintenance loss within a fluorescent lamp. This behavior is probablyassociated with the reaction of the phosphor with the mercury dischargewithin the fluorescent lamp.

Shown in FIG. 5 is a cross-sectional view of a phosphor particle coatedwith a bi-layer. The phosphor grain is coated with a silica layer whichprevents diffusion from the phosphor grain to the surface coating ofalumina. It is also believed that the silica layer prevents diffusion ofthe alumina layer to the phosphor grain. Potential uses of the phosphorshown in FIG. 5 are discussed below.

Historically silica and silica-containing phosphors used withinfluorescentlamps are known to give rise to appreciable maintenance loss,the uncoated willemite, in fact, being a prime example of this.Therefore, any improvement due to a protective layer over a silicacoating can be expected to be strongly dependent on the quality andconformality of that layer as well as the intrinsic resistance of thespecific phosphor to degradation in a fluorescent lamp.

EXAMPLE 4

Silica coatings were applied to the surfaces of zinc silicate phosphorsby CVD in a fluidized bed described in Example 1. However, the testswere carried out using typically 1500 gms. of phosphor in an 80 mm IDquartz tube which employed a quartz frit as the distributor. AluminumOxide C wasblended with the phosphor at a concentration of 0.1% byweight of the fluidized bed reactor was maintained between 450° and 460°C. during the coating process. This temperature was monitored by athermocouple placed within the bed located at the midbed height. In atypical run 2 liters per minute were run through a bubbler containingtetramethoxyorthosilicate (TMOS) liquid maintained at 32° C., and3liters per minute of undiluted oxygen entered the bed through a hollowstirrer rod which was located a few centimeters above the level of theporous distributor plate. Coating reactions were carried out between11/2 hrs. and 5 hrs. to effect the deposition of predetermined amountsof silica coating. Table 7 summarizes the powder properties of thevirgin phosphors used in the following examples.

                  TABLE 7                                                         ______________________________________                                        Particle Properties of the Virgin Zinc Silicate                               Phosphors Used in the Examples Cited                                                            FSS         Coulter Counter                                 Lot    Surface Area                                                                             (Fisher Sieve                                                                             Particle Size (sonic) .sup.2                    No.    (M.sup.2 /gm) .sup.1                                                                     Size, microns)                                                                            50%    Q.D..sup.3                               ______________________________________                                        66RMF  0.38       6.7         9.0    0.23                                     TK1-2M 0.44       6.1         7.72   0.26                                     TK2-U  0.52       5.1         6.65   0.27                                     ______________________________________                                         .sup.1 Determined by singlepoint BET measurements, using a Quantachrome       Monosorb surface area instrument.                                             .sup.2 Based on volume.                                                       .sup.3 Q.D. is defined as (d.sub.75% - d.sub.25%)/(d.sub.75% + d.sub.25%)    and is a relative measure of breadth in the particle distribution.        

Following coating, the phosphors were annealed in quartz boats for about4 hours at an annealing temperature of approximately 760° C. At thispoint phosphors were coated with alumina as described below.

The CVD coatings were carried out on the annealed phosphors describedabovein a fluidized bed using trimethyl aluminum (TMA) and oxygen asprecursors.The equipment and procedures for fluidized bed coating ofwillemite phosphors are described in detail in U.S. patent applicationSer. No. 07/406,884 now U.S. Pat. No. 4,950,948. Briefly, a blend ofapproximately 1000 to 1300 gms. and 0.1% of Aluminum Oxide C by weightof phosphor was loaded into a quartz fluidized bed column comprising an80 mm ID quartz tube having a quartz frit fused to the bottom whichacted as the distributor plate. A 65 mm stainless steel agitator discwas attached to avibromixer agitator. Approximately 5 cm from the base,a two-micron stainless steel filter element was welded in line andfunctioned as a diffuser of the oxygen mixture. The agitator disc itselfwas located approximately 25 mm above the quartz distributor. Athermocouple, located at the midbed height within the fluidized bed, wasused to monitor the temperature of the bed, which was maintained betweenapproximately 420° and 450° C.

The apparatus used for carrying out the coating reactions is shown inFIG. 1 with some slight alterations. In a typical run, which lastedbetween 3 and 5 hours, 1750 cc per minute of nitrogen passed through abubbler 12 containing trimethyl aluminum (TMA) liquid which wasmaintained at 300° C. Another 1250 cc per minute of nitrogen carrier gaswas usedto dilute the flow through line 111. The combined flow was usedto fluidizethe phosphor particles in reaction vessel 15. The oxygen asan oxygen/inertgas mixture was introduced through line 21 at 2500 cc perminute of oxygen and 50 cc per minute of nitrogen into the fluidized bedthrough the two-micron filter element described earlier. Table 8summarizes specific coating parameters used for the examples cited.

                                      TABLE 8                                     __________________________________________________________________________    Coating parameters used to apply Al.sub.2 O.sub.3 and SiO.sub.2 layers to     the                                                                           zinc silicate phosphors described in this disclosure.                              Sample                                                                             Run   Bed  Bub-    Coating                                                                            Run     Bed                                 No.  No.  No.   Loading                                                                            bler                                                                              O.sub.2                                                                           Time No.     Loading                             __________________________________________________________________________    6RMFF                                                                              #385 No silica               CWM120-89                                                                             2000 gms                                 #441 TMOS-29L                                                                            1500 gms                                                                           2 l/min                                                                           3 l/min                                                                           31/2 hrs.                                                                          CWM330-89                                                                             1230 gms                            k1-2M                                                                              #518 No silica               CWM1017-89                                                                            1500 gms                                 #479 TMOS-38L                                                                            1500 gms                                                                           2 l/min                                                                           3 l/min                                                                           11/2 hrs.                                                                          CWM614-89                                                                             1295 gms                                 #476 TMOS-37L                                                                            1500 gms                                                                           2 l/min                                                                           3 l/min                                                                            5 hrs.                                                                            CWM612-89                                                                             1070 gms                            k2-U #425 No silica               CWM313-89                                                                               350 gms+                               #443 TMOS-30L                                                                            1500 gms                                                                           2 l/min                                                                           3 l/min                                                                            5 hrs.                                                                            CWM403-89                                                                             1120 gms                            __________________________________________________________________________                                SiO.sub.2 Al.sub.2 O.sub.3                             Sample                                                                             Bub-                                                                              Car-                                                                                  Coating                                                                             Wt        Wt                                      No.  No.  bler                                                                              rier                                                                              O.sub.2                                                                           Time  %   Thickness.sup.#                                                                     %   Thickness.sup.#                     __________________________________________________________________________    6RMFF                                                                              #385 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           5  hrs.         2.28%                                                                             150Å                                 #441 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           3  hrs.                                                                             1.26%                                                                             144Å                                                                            2.22%                                                                             147Å                            k1-2M                                                                              #518 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           41/3                                                                             hrs.             150Å                                 #479 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           43/4                                                                             hrs.                                                                             0.81%                                                                              80Å                                                                            2.65%                                                                             150Å                                 #476 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           31/2                                                                             hrs.                                                                             1.80%                                                                             178Å                                                                            2.62%                                                                             150Å                            k2-U #425  400 cc                                                                            350 cc                                                                            500 cc                                                                           4  hrs.             115Å                                 #443 1750 cc                                                                           1250 cc                                                                           2500 cc                                                                           3 5/6                                                                            hrs.                                                                             1.80%                                                                             150Å                                                                            3.12%                                                                             150Å                            __________________________________________________________________________     .sup.+ This run was carried out in a 40 mm I.D. quartz column; earlier        data has shown equivalence between the small (40 mm) and large (80 mm)        column runs. Also, no performance differences have been seen between          100-300Å Al.sub.2 O.sub.3 -only coated willemite phosphors.               .sup.# Values for the thickness are derived from the weight percent           SiO.sub.2 or Al.sub.2 O.sub.3 deposited and an assumed density of 2.3         gms/cc and 3.97 gms/cc, respectively.                                    

Once the phosphor particles were coated, they were transferred to quartzboats and annealed at approximately 760° C. for 4 hours.

Lamp Testing

After the annealing step described above, the phosphors were coated in20WT12 or 30 WT12 fluorescent lamps using conventional water basesuspension systems. The lamps thus coated were processed into finishedfluorescent lamps and photometrically evaluated. The data were thenconverted to corresponding 40WT12 data using established correctionfactors.

In the tests cited below, the bilayer coated and annealed phosphors weretested against Sylvania Type 2293, which is a Ce,Tb, Mg Aluminatephosphorused as a green-emitting component in high color renditionlamps. In addition, the uncoated virgin phosphor and the singly coatedand annealed (i.e., alumina only) phosphors were also evaluated with thetest.

Table 9 lists the lifetest results of the phosphors. It is clearlyevident that the bilayer coating provides 0-hr. brightness values whichare eitherequivalent to or exceeds that of the virgin phosphor lot.Also, both the single- and bilayer-coated phosphors possess maintenancewhich significantly exceeds the virgin phosphor. However, the bilayer(Al₂ O₃ /SiO₂) annealed phosphor substantially exceeds the brightnessofthe Al₂ O₃ -only phosphor by 8% for the `RMF` phosphor (66RMF)to as muchas 17% for the singly fired willemite phosphors (TK1-2M and TK2-U lots).Note that the brightness observed for the alumina-only coatedandannealed singly fired willemite phosphor lots are so low as to eliminatetheir consideration for commercial use in conventional high colorrendition lamps, whereas the substantially higher brightness of the Al₂O₃ /SiO₂ coated singly fired phosphor allows its use intriblend lamps.The thickness of the silica coatings are derived from the informationshown in Table 8.

                                      TABLE 9                                     __________________________________________________________________________    Lifetest Data for Various Willemite Phosphor                                             0 hr                                                                             100 hr                                                                            % M(0-100)                                                                           500 hr                                                                            % M(0-500)                                       __________________________________________________________________________    66RMF Lot                                                                     Virgin     5329                                                                             3800                                                                              71.3   Discontinued                                         #385 Al.sub.2 O.sub.3                                                                    4860                                                                             4763                                                                              98.0   4685                                                                              96.4                                             #441 Al.sub.2 O.sub.3 /SiO.sub.2                                                         5245                                                                             5101                                                                              97.3   5048                                                                              96.2                                             FHX164 Type 2293                                                                         4923                                                                             4768                                                                              96.8   4685                                                                              95.2                                             TK1-2M Lot                                                                    Virgin     5199                                                                             --  --     --  --                                               #578+ Al.sub.2 O.sub.3                                                                   4491                                                                             4424                                                                              98.5   --  --                                               #479 Al.sub.2 O.sub.3 /                                                                  5219                                                                             5193                                                                              99.5   4887                                                                              93.6                                             SiO.sub.2 (80Å)                                                           #476 Al.sub.2 O.sub.3 /                                                                  5239                                                                             5137                                                                              98.0   4894                                                                              93.4                                             SiO.sub.2 (178Å)                                                          FHX343 Type 2293                                                                         4873                                                                             4757                                                                              97.6   4499                                                                              92.3                                             TK2-U Lot                                                                     Virgin     5021                                                                             --  --     --  --                                               #425 Al.sub.2 O.sub.3                                                                    4499                                                                             4332                                                                              96.3   4264                                                                              84.7                                             #443 Al.sub.2 O.sub.3 /SiO.sub.2                                                         5084                                                                             4878                                                                              95.9   4842                                                                              95.2                                             FHX164 Type 2293                                                                         4923                                                                             4768                                                                              96.8   4685                                                                              95.2                                             __________________________________________________________________________     +This sample was run in a separate test against Type 2293 Lot FHX343. In      that test FHX343 gave 0 hrs = 4903 l, 100 hrs = 4736 l, (0-100% M =           96.6%). Therefore, the major conclusions put forth are not affected.         Note that values given have been corrected from corresponding 20WT12 and       30WT12 to 40WT12 fluorescent lamp data using established correction           factors. Test Samples for lots 66RMF and TK2U were evaluated in 20WT12        lamps; Lot TK12M was evaluated in 30WT12 lamps.                          

As mentioned earlier, the singly fired phosphor can be manufactured on aproduction scale with much higher yields than can be obtained using the`RMF` synthesis described by Kasenga in our commonly assigned U.S.patent application 07/434,741 which is still pending. The much higheryields and efficiencies of scale favor substantial cost savings.

It is important to note that the addition of the silica interfaceprovides a major improvement in the 100-hr. brightness, as well, overthe alumina coatings alone. As the data in Table 9 show, Sample #479 hasyielded a 100-hour brightness of 5193 lumens. This corresponds to thehighest value ever achieved for a willemite phosphor after 100 hours ofburning corrected to equivalent 40WT12 fluorescent lamp performance.This has significant commercial implications since published ratings of`initial brightness` are actually those determined after 100 hours oflamp operation.

When tricomponent blend suspensions are used which consist of Y₂ O₃ :Eufor the red emission, Ba,Mg Aluminate:Eu for the blue emission, and the`RMF` coated and annealed phosphor described by our commonly assignedU.S. patent application 07/434,741 for the green emission, a colorvariation in the emitted light is observed from the fluorescent lampacross its length manifested by a slightly red colorationon the morethinly coated end of the lamp and slightly green coloration on the morethickly coated end of the fluorescent lamp. This end-to-endcolorvariation is believed to be due, in part, to the disparity inparticle sizebetween the red and green phosphors in the suspension usedto make the coated lamps. In fact, lamp fabrication using Sample #443, abilayer coated and annealed TK2-U lot of smaller particle size than the`RMF` phosphor, showed a significant reduction in color nonuniformitycompared to a coated and annealed 66 RMF phosphor (Sample #441) run inthe same test. Thus, the bilayer coating allows particle size reductionwhile maintaining excellent brightness for high color rendition triblendapplications.

Table 10 lists the x-ray photoelectron spectroscopy (XPS) analyses ofthe surfaces after each of the stages of processing leading to thealumina only and alumina/silica coated and annealed phosphors. The dataclearly show that the silica layer either eliminates or substantiallyreduces the migration of Zn and Mn through the alumina coating. It isalso expected, although not experimentally verified, that the silicainterfacial layer also prevents the migration of Al ions from thealumina coating into the zinc silicate phosphor. Both factors contributeto the elimination of undesirable light-absorbing body color of theannealed phosphor and the interference from impurities that lead to theloss in generation of efficient luminescence.

                  TABLE 10                                                        ______________________________________                                        X-ray Photoelectron Spectroscopy Analyses of                                  Coated Willemite Phosphors                                                                Atomic Percent                                                    Sample Designation                                                                          Sl     Al     O    Zn    Mn   C                                 ______________________________________                                        66RMF Virgin Powder                                                                         15.8   --     54.6  22.3 0.7  6.6                               CWM 120-89 (TMA                                                                             --     43.4   51.6 <0.1  --   4.9                               coated)                                                                       #385 (TMA coated/                                                                           --     42.0   52.5  4.3  0.2  1.1                               annealed)                                                                     TM0S 29L (Silica                                                                            38.5   --     58.8  1.1  --   1.4                               coated)                                                                       #439 (Silica coated/                                                                        39.0   --     58.2 --    --   2.8                               annealed)                                                                     CWM 330-89 (TMA                                                                             --     44.1   51.6 --         4.3                               coated #439)                                                                  #441 (CWM 330-89/                                                                           --     45.2   51.3  0.2  --   3.2                               annealed)                                                                     TK-2U Virgin phosphor                                                                       16.0   --     48.6  17.7 0.9  16.0                              CWM 313-89 (TMA                                                                             --     43.7   52.8 --    --   3.5                               coated)                                                                       #425 (TMA coated/                                                                           --     36.9   45.8  10.1 0.4  6.6                               annealed)                                                                     TMOS 30L (Silica                                                                            36.4   --     56.1  0.2  --   7.3                               coated)                                                                       #440 (Silica coated/                                                                        37.2   --     53.9 <0.1  --   8.8                               annealed)                                                                     CWM 403-89 (TMA                                                                             --     43.6   52.8 --    --   3.6                               coated #440)                                                                  #443 (CWM 403-89                                                                            --     45.4   50.4 <0.1  --   4.2                               annealed)                                                                     TK1-2M Virgin 13.3   --     44.1  27.7 0.5  14.3                              phosphor                                                                      TMOS 38L (Silica                                                                            38.1   --     60.4 <0.1  --   1.5                               coated)                                                                       #477 (Silica coated/                                                                        37.7   --     59.8 <0.1  --   2.4                               annealed)                                                                     CWM 614-89 (TMA                                                                             --     42.5   53.1 --    --   4.4                               coated #477)                                                                  #479 (CWM 614-89                                                                            --     45.1   51.8 <0.1  --   3.0                               annealed)                                                                     ______________________________________                                    

Lamp tests carried out employing the single-coated alumina `RMF` zincsilicate phosphor produced by the Chemical and Metallurgical Division ofGTE Products Corporation, Towanda, Pa., have shown a lower brightnesslevel in triphosphor blends when compared with the rare-earth-containingblends run in the same test (See Table 11). A bilayer (Al₂ O₃ /SiO₂)coated willemite described employing a single-step fired willemite basematerial in 40WT12 fluorescent lamps (Sample #541) has yieldedperformance of 5280 lumens at 0 hours of lamp operation and 5144 lumensat 100 hours with a maintenance of 97.4% which exceeds that of the Type2293 by almost 1%. This performance substantially exceeds that obtainedwith the best alumina-coated "RMF" materials heretofore available. (SeeTable 11 for single-component life test data) Thus, it is probable thatthis substantially less expensive silica/alumina coated single-stepfired willemite phosphor is useful as single components and ascomponentsof blends in double twin-tube lamps. This phosphor will replace the moreexpensive Sylvania Type 2293 and the Nichia LaPO₄ :Ce, Tb phosphorspresently used. Further uses for the silica/alumina coated phosphors arediscussed below.

                                      TABLE 11                                    __________________________________________________________________________    Lifetest data for lamps containing alumina-coated `RMF`                       willemite and rare-earth green tri-phosphor blends.*                          __________________________________________________________________________    Single-Coat Lamps: 20WT12, 3500° K.                                                              Lumens                                              Green Test Sample                                                                          P.Wt.(gms)                                                                           X  Y  0 Hr                                                                              100 Hr                                                                            (0-100 Hr) % M                              __________________________________________________________________________    Control 2293 1.95   0.410                                                                            0.398                                                                            1433                                                                              1389                                                                              96.9                                        G077(Willemite)                                                                            1.85   0.408                                                                            0.397                                                                            1354                                                                              1308                                                                              96.6                                        Delta Lumens              -79 -81                                             Delta Percent             -5.5%                                                                             -5.8%                                           Double-Coat Lamps: 20WT12, D35 (Designer, 3500° K.)                                 Second Coat  Lumens                                              Green Test Sample                                                                          P.Wt.(gms)                                                                           X  Y  0 Hr                                                                              100 Hr                                                                            (0-100 Hr) % M                              __________________________________________________________________________    Control 2293 0.52   0.412                                                                            0.400                                                                            1353                                                                              1339                                                                              99.0                                        G077(Willemite)                                                                            0.55   0.412                                                                            0.398                                                                            1314                                                                              1291                                                                              98.2                                        Delta Lumens              -39 -48                                             Delta Percent             -2.9%                                                                             -3.6%                                           __________________________________________________________________________    *Lifetest data for single components in 40WT12 fluorescent lamps are:                      0 Hr                                                                             100 Hr                                                                            (0- 100 Hr) % M                                           Control 2293 4921                                                                             4740                                                                              96.3                                                      G077 (60RMF) 5019                                                                             4695                                                                              93.5                                                      Virgin 60RMF(uncoated)                                                                     5134                                                                             4143                                                                              80.7                                                  

Referring to FIGS. 6 and 7, there is shown in FIG. 6 and arc dischargelampof the fluorescent type. The lamp 10 is comprised of an elongatedglass tube 12 of circular cross-section. It has the usual electrodes 14and 16 at each end supported by lead-in wires, 18, 20 and 22, 24,respectively, which extend through glass presses 26, 28 in mount stems30, 32 to the contacts in bases 34, 36 affixed to the ends of the lamp.

The sealed tube is filled with an inert gas such as Argon or a mixtureof Argon and Krypton at a low pressure, for example 2 torr, and a smallquantity of mercury, at least enough to provide a low vapor pressureduring operation.

The interior of tube 12 is coated with a first layer of phosphor 38 suchas, for example, a calcium halophosphate activated by antimony andmanganese.

A phosphor coating suspension was prepared by dispersing the phosphorparticles in a water-based system employing polyethylene oxide andhydroxyethyl cellulose as the binders with water as the solvent.

The phosphor suspension was applied in the usual manner of causing thesuspension to flow down the inner surface of the bulb and allowing thewater to evaporate leaving the binder and phosphor particles adhered tothe bulb wall.

The first layer 38 was then dried prior to overcoating with a secondphosphor layer 40 comprised of narrow-band red- and blue- emittingphosphors and a broad-band green-emitting phosphor. These twonarrow-band phosphors can be, for example, a yttrium oxide activated bytrivalent europium and having a peak emission at 611 nm; and bariummagnesium aluminate activated by divalent europium and having a peakemission at 455nm. The broader band phosphor was alumina/ silica-coatedzinc silicate activated by manganese and having a peak emission at 528nm.

The second phosphor layer containing the CVD-coated phosphor is appliedfrom a water-based suspension by allowing the coating to flow down overthe first phosphor layer 38 until the phosphor coating drained from thebottom of the bulb indicating the coverage of the phosphor layer 38 wascomplete. Lamps made by this method usually exhibit the thinnest coatingthickness on the top end of the bulb and the heaviest thickness on thebottom end, where the suspension is allowed to drain. The double-coatedbulbs were then baked and processed into fluorescent lamps byconventionaltechniques.

In the case where only a single layer lamp was made, the methods wereessentially the same as described herein with the exception that thehalophosphate layer was not applied.

Control lamps were fabricated by identical techniques as described abovebut had a narrow-band green-emitting magnesium aluminate phosphoractivated by cerium and terbium in the second phosphor layer with a peakemission at 545 nm. This phosphor is generally used in the tri-phosphorblend but was replaced by the green CVD-coated willemite phosphor inthis invention.

Lamps employing a representative alumina-only coated and annealed `RMF`willemite phosphor, manufactured by the Chemical and MetallurgicalDivision of PMG, Towanda, PA, were also fabricated and incorporated intothe testing.

Lifetest data and particle size information for the single componentwillemite phosphors used for the tri-phosphor blend lamps are given inTables 12 and 13, respectively. These values have been corrected toobtainthe performance levels that would be observed with 40T12fluorescent lamps using established correction factors.

                  TABLE 12                                                        ______________________________________                                        Lifetest data for single-component fluorescent lamps                          containing coated willemite phosphors (corrected to 40WT12)                              0 Hr 100 Hr   500 Hr  % M(0-500 Hr)                                ______________________________________                                        Type 2293 (control)                                                                        4923   4768     4685  95.2                                       #425 (TK2-U)+                                                                              4499   4332     4264  94.7                                       #443 (TK2-U)++                                                                             5084   4878     4842  95.2                                       G103 (61RMF)+                                                                              5085   4804     4677  92.0                                       #441 (66RMF)++                                                                             5245   5101     5048  96.2                                       ______________________________________                                         +Phosphor grains coated with only a single coating of alumina, (sample        G103 was prepared at Chem. and Met. Div. PMG, Towanda, PA).                   ++Phosphor grains coated with both silica and alumina.                   

                  TABLE 13                                                        ______________________________________                                        Particle properties of the virgin zinc silicate                               phosphors used in the examples cited.                                                                Coulter Counter                                               FSS             Particle Size(sonic)                                   Lot Number                                                                             (Fisher Sieve Size, microns)                                                                    50%      Q.D.                                      ______________________________________                                        Singly Fired                                                                  TK-U     5.1               6.7      0.27                                      `RMF`                                                                         60RMF    7.4               9.3      0.21                                      61RMF    8.0               10.0     0.21                                      66RMF    6.7               9.0      0.23                                      ______________________________________                                    

For the evaluation of the tri-blends containing thealumina/silica-coated willemite phosphor, two different lamp types wereused, and compared with controls which did not have this phosphor in theblend. The lamps were tested by photometering for light output in astandard photometric sphere,both initially and at stated times. In thefollowing tables, light outputs are expressed in lumens. Lamp colorvalues were obtained by spectral powerdistribution (SPD) measurements.

EXAMPLE 5

This example compares the light output and maintenance ofalumina/silica-coated and annealed willemite phosphors in thetri-phosphorblends for double layered 96-inch Designer 3000°K HighOutput Super Saver fluorescent lamps. This lamp type has a tri-phosphorweight in the second layer of about 15% of the total phosphor in the twolayers. The lamps were fabricated to obtain the same x and y colorcoordinates for both the test and control by adjusting the tri-phosphorblend composition.The lamp test results are listed in Table 14. As thetest data show, the lumens and lumen maintenance of the test group areequivalent to the rare-earth-containing Type 2293control. This is incontrast to previous results obtained with alumina-only (single-coatedand annealed) `RMF` phosphor where Designer lamp brightness values weretypically 3% to 4% lower than those obtained with the standardtri-phosphor blend (see Table 11 for comparison).

                                      TABLE 14                                    __________________________________________________________________________    Lifetest data for double-coat lamps where the triblends contain               rare-earth                                                                    green-emitting phosphor and two Al.sub.2 O.sub.3 /SiO.sub.2 -coated           willemite phosphors.                                                          Lamp Type: 96T12/D30/HO/SS (Designer, 3000° K., high output,           supersaver)                                                                   Green                 Composition-Second Layer                                                                             Tri-Phosphor                     Component             Tri-Phosphor (Weight %).sup.+ (1)                                                                    Weight Color                                                                             Coordinates           Test Sample                                                                           Phosphor      Red(Y.sub.2 O.sub.3 :Eu)                                                              Blue(Ba,MgAlO:Eu)                                                                        Green                                                                             (Grams)                                                                              X   Y                     __________________________________________________________________________    Control 2293                                                                          (Ce,Tb)MgAlO:Ce,Tb                                                                          65.8    2.9        31.3                                                                              1.84   0.429                                                                             0.410                 #441(66RMF)                                                                           Zn.sub.2 SiO.sub.4 :Mn,W`RMF`*                                                              66.7    2.5        30.8                                                                              1.90   0.435                                                                             0.405                 #443(TK2-U)                                                                           Zn.sub.2 SiO.sub.4 :Mn,W Single Fired                                                       66.1    2.5        31.4                                                                              1.92   0.434                                                                             0.403                 __________________________________________________________________________     .sup.+ First layer is a holophosphate phosphor Sylvania Type 4300 with        color coordinates in finished lamp of X = 0.440, Y = 0.405; first layer       weight = 11-12 gms.                                                          *Alumina/silica coated                                                    

    Photo Metric Results                                                                 Lumens                                                                             Lumens                                                                              Lumens                                                                              Lumen Maintenance %                                          0 Hour                                                                             100 Hours                                                                           500 Hours                                                                           0-500 Hours                                           Control 2293                                                                         8753 8573  8060  92.1                                                  #441   8763 8430  8082  92.2                                                  #443   8765 8368  8063  92.0                                                  (1) Red component is Sylvania Type 2345 (Y.sub.2 O.sub.3 :Eu); Blue            component is Sylvania Type 246 (Ba,MgAlO:Eu)                             

EXAMPLE 6

As mentioned earlier, color nonuniformities of fluorescent lampscontainingthe `RMF` coated and annealed phosphor occur and become moresevere as the bulb length increases. The 96-inch lamps have shown themost pronounced variation. Color nonuniformities strongly influenceconsumer acceptance ofthe lamp since they are a premium-priced productdesigned for use in high color rendition applications.

FIGS. 8 through 10 show the color points taken from the more heavilycoatedend (HE), middle (M), and more lightly coated end (LE) of thefluorescent lamp containing the rare earth green, the bilayer-coated`RMF` phosphor (#441), and the bilayer-coated single-fired willemitephosphor (#443), respectively. Also shown are the two- and three-stepMacadam ellipses. TheMacadam ellipse is a way of assessing differencesin visual color perception. For acceptable lamps it is desirable to haveall points residing well within the two-step ellipse. As shown in FIG.8, the end-to-end color variation is excellent in the case of therare-earth green Type 2293. However, the `RMF`-based phosphor shows ared and green variation along the length of the lamp, as shown in FIG.9. In contrast, as shown in FIG. 10, the singly fired phosphor (Sample#443) shows a significant improvement in color uniformity over the `RMF`phosphor, well within acceptable limits. It is thought that the colornon-uniformity originates from a particle-size disparity between the redand green phosphors that make up the dispersion from which the lamps aremade. Thus,the bilayer coating allows particle size reduction of thegreen component while maintaining excellent brightness for high colorrendition applications. (As shown in Table 12, the fluorescent lampbrightness obtained with the alumina/silica-coated small particlephosphor (Sample #443) is much superior to that obtained with the samebase phosphor coatedonly with alumina (Sample #425)).

EXAMPLE 7

This example compares the light output and maintenance of alumina-coatedand alumina/silica-coated willemite phosphors in 40WT12 4100°Ksingle-coat tri-phosphor lamps. This test is designed to exaggeratedifferences in performances between the green components used in thetriblends, since the single layer of high color temperature requires thelargest amount of green compared to any other lamp that would befabricated. That is, any differences in performance will diminish as thetriblend layer thickness is reduced (in double layer lamps) and as thelamp color temperature is reduced (since the fraction of green componentin the blend goes down as the color temperature is reduced).

As the data in Table 15 show, the maintenance of all materials testedare comparable. Further, the color rendering index, measured after 100hours of lamp operation, is about 3 units higher for thewillemite-containing blends compared to the rare-earth-containing blend,even in this single-coat lamp, achieving CRI values in excess of 85.

                                      TABLE 15                                    __________________________________________________________________________    Lifetest data for triblends containing rare-earth green-emitting green        phosphor,                                                                     alumina-coated willemites, and Al.sub.2 O.sub.3 /SiO.sub.2 -coated            willemite phosphors.                                                          Lamp Type: 40T12/4100° K./single-coat triphosphor (no halo layer)      Green                Composition-Second Layer                                                                             Tri-Phosphor                                                                             Coor-                                                                             CRI                Component            Tri-Phosphor (Weight %)*                                                                             Weight Color                                                                             dinates                                                                           100                Test Sample                                                                            Phosphor    Red(Y.sub.2 O.sub.3 :Eu)                                                              Blue(Ba,MgAlO:Eu)                                                                        Green                                                                             (Grams)                                                                              X   Y   Hours              __________________________________________________________________________    Control 2293                                                                           (Ce,Tb)MgAlO:Ce,Tb                                                                        48.0    10.3       41.6                                                                              3.31   0.376                                                                             0.385                                                                             82.5               G103(61RMF)**                                                                          Zn.sub.2 SiO.sub.4 :Mn,W                                                                  51.4    11.4       37.2                                                                              3.85   0.376                                                                             0.385                                                                             85.6                        RMF Al.sub.2 O.sub.3 only                                            #441(66RMF)***                                                                         Zn.sub.2 SiO.sub.4 :Mn,W RMF                                                              54.8    11.6       33.6                                                                              4.07   0.380                                                                             0.389                                                                             85.3               #443(TK2-U)***                                                                         Zn.sub.2 SiO.sub.4 :Mn,W                                                                  53.1    12.6       34.3                                                                              3.74   0.378                                                                             0.388                                                                             85.4                        Single Fired                                                         __________________________________________________________________________    *Red component is Sylvania Type 2345 (Y.sub.2 O.sub.3 :Eu); Blue component     is Sylvania Type 246 (Ba,MgAlO:Eu)                                           **Manufactured at Chemical & Metallurgical Division, PMG, Towanda, PA          (Aluminacoated willemite phosphor)                                           ***Alumina/Silica-coated willemite phosphor.                                  Photo Metric Results                                                                  Lumens                                                                             Lumens                                                                              Lumens                                                                              Lumens                                                                              Lumen Maintenance %                                    0 Hour                                                                             100 Hours                                                                           500 Hours                                                                           1000 Hours                                                                          0-1000 Hours                                   Control 2293                                                                          3427 3319  3252  3161  92.2                                           G103(61RMF)                                                                           3341 3183  3080  3006  90.0                                           #441(66RMF)                                                                           3421 3291  3215  3176  92.8                                           #443(TK2-U)                                                                           3364 3233  3147  3095  92.0                                       

With regard to brightness, the bilayer-coated `RMF` phosphor (#441) isclearly superior to the representative singly coated `RMF` phosphor(G103). The brightness performance of the bilayer-coated singly firedzincsilicate (#443), while exceeding that of the alumina-coated `RMF`phosphor (G103), is about 2% below that of the rare-earth Type 2293control, at 1000 hours of lamp operation. However, by way of comparisona singly fired(smaller particle) willemite phosphor with only thealumina coating (i.e., without the intervening layer of silica) iscompletely unsuitable for use in triblend applications because itsbrightness is about 10% below that ofthe correspondingalumina/silica-coated phosphor (i e., less than 4500 lumens initialbrightness in a 40WT12 fluorescent lamp), as shown in Table

The use of the bilayer-coated phosphor provides a substantial costsavings over the rare-earth blends in the examples cited above since thetri-phosphor blend represents the major cost of the lamp and, as Table14 shows, the green component comprises over 30% of the tri-phosphorblend.

Further, the use of the singly fired bilayer-coated phosphor provideseven further cost savings, since the large-particle `RMF` material withnarrow particle size distribution (described in our commonly assignedU.S. patentapplication 07/434,741) requires two firings in its synthesiswith multipledecantations to remove the "fines" fraction. Thisnecessarily results in low yields (typically 60%). However, the smallerparticle material can be made, as described by Chenot, using asingle-step firing followed by a washing which provides much higheryields (typically 90%). Also, the smaller particle material can easilybe manufactured on production-scale equipment. The much higher yieldsand the efficiencies of scale all favor substantial cost savings whichfar outweigh the cost of applying the intermediate silica layer.

It is evident that fluorescent lamps will benefit greatly from the useof the bilayer-coated and annealed singly fired willemite phosphor bypermitting a lower lamp price that finds more acceptance in themarketplace.

Finally, while what has been described herein has been fluorescent lampsemploying CVD-coated phosphors in single- and double-layeredconfigurations, the scope of this disclosure can include lamps whichemploy multiple layers of phosphor coatings in the fabrication of thelamp, multiple components of the blend in addition to, or otherwisedifferent from, the tri-phosphor blend formulation described herein solong as they contain the bilayer-coated willemite as one of thecomponents, and the use of non-CVD-coated alumina/silica-coated andannealed willemite phosphor.

While there has been shown and described what are at present consideredto be the preferred embodiments of the invention, it will be obvious tothoseskilled in the art that various changes and modifications can bemade without departing from the scope of the invention as defined by theappended claims.

What is claimed:
 1. A method for forming a continuous coating inphosphor particles in a fine phosphor powder comprising:a) vaporizing asilicon containing precursor material selected from the group consistingof tetramethyloxysilane and tetraethyloxysilane into an inert carriergas to form a carrier gas containing vaporized silicon precursor; b)passing said carrier gas containing silicon precursor through a mixtureof phosphor powder and up to 1 percent fluidizing aid to form afluidized bed in which the particles are suspended in the carrier gasand to envelope the fluidized particles with vapor of the siliconprecursor said fluidized bed being maintained at a nearly isothermalcondition and at a temperature above the decomposition temperature ofthe silicon-containing precursor; c) passing an oxidizing gas into saidfluidized bed separately from said carrier gas containing vaporizedsilicon precursor and reacting said oxidizing gas with the vaporizedsilicon precursor on the particles of the phosphor powder to form acontinuous coating of silica of predetermined thickness on the phosphorparticles; d) annealing the particles of the phosphor obtained from step(c); e) vaporizing an aluminum-containing precursor material selectedfrom the group consisting of aluminum isopropoxide andtetramethylaluminum into an inert carrier gas to form a carrier gascontaining vaporized aluminum-containing precursor material; f) passingsaid carrier gas containing vaporized aluminum precursor materialthrough the phosphor powder having a continuous coating of silica asproduced by step (c) to form a fluidized bed in which particles aresuspended in the carrier gas containing vaporized aluminum precursormaterial and to envelope the fluidized particles with vapor of aluminumcontaining precursor material said fluidized bed being maintained at anearly isothermal condition, and at a temperature above thedecomposition temperature of the aluminum-containing precursor; g)passing an oxidizing gas into said fluidized bed of step (f) separatelyfrom said carrier gas containing vaporized aluminum-containing precursormaterial and reacting said oxidizing gas with the vaporizedaluminum-containing precursor material on the particles of the phosphorpowder having a continuous coating of silica to form a continuouscoating of alumina of predetermined thickness on the phosphor particleshaving a continuous coating of silica.
 2. The method according to claim1 further comprising:h) annealing the particles of the phosphor obtainedfrom step (g).
 3. The method according to claim 1 wherein the phosphorparticles comprise a manganese activated zinc silicate phosphor.
 4. Themethod according to claim 1 wherein the phosphor particles comprise acalcium halophosphate phosphor.
 5. A method for forming a continuousbi-layer coating in phosphor particles in a phosphor powdercomprising:a) vaporizing a silicon-containing precursor materialselected from the group consisting of tetramethyloxysilane andtetraethyloxysilane into an inert carrier gas to form a carrier gascontaining vaporized silicon-containing precursor b) passing saidcarrier gas containing silicon-containing precursor through a mixture ofphosphor powder and up to 1 percent fluidizing aid to form a fluidizedbed in which the particles are suspended in the carrier gas and toenvelope the fluidized particles with vapor of silicon-containingprecursor material, said fluidized bed being maintained at a nearlyisothermal condition and a temperature above the decompositiontemperature of the silicon-containing precursor material; c) passing anoxidizing gas into said fluidized bed separately from said carrier gascontaining vaporized silicon-containing precursor and reacting saidoxidizing gas with the vaporized silicon-containing precursor on theparticles of the phosphor powder to form a continuous coating of silicaof predetermined thickness on the phosphor particles; d) vaporizing analuminum-containing precursor material selected from the groupconsisting of aluminum isopropoxide and tetramethylaluminum into aninert carrier gas to form a carrier gas containing vaporizedaluminum-containing precursor material; e) passing said carrier gascontaining vaporized aluminum precursor material through the phosphorpowder having a continuous coating of silica as produced by step (c) toform a fluidized bed in which particles are suspended in the carrier gascontaining vaporized aluminum precursor material and to envelop thefluidized particles with vapor of aluminum-containing precursor materialsaid fluidized bed being maintained at a nearly isothermal condition andat a temperature above the decomposition temperature of the aluminumcontaining precursor; and f) passing an oxidizing gas into saidfluidized bed of step (e) separately from said carrier gas containingvaporized aluminum-containing precursor material and reacting saidoxidizing gas with the vaporized aluminum-containing precursor materialon the particles of the phosphor powder having a continuous coating ofsilica to form a continuous coating of alumina of predeterminedthickness on the phosphor particles having a continuous coating ofsilica.
 6. The method according to claim 5 further comprising:h)annealing the particles of phosphor obtained from step (g).
 7. Themethod according to claim 5 wherein the phosphor comprises amanganese-activated zinc silicate phosphor.
 8. The method according toclaim 5 wherein the phosphor comprises a calcium halophosphate phosphor.9. A method for forming a continuous coating on phosphor particles in afine phosphor powder comprising:a) vaporizing a silicon containingprecursor material selected from the group consisting oftetramethyloxysilane and tetraethyloxysilane into an inert carrier gasto form a carrier gas containing vaporized silicon-containing precursormaterial; b) passing said carrier gas containing silicon-containingprecursor material through a mixture of phosphor powder and up to 1percent fluidizing aid to form a fluidized bed in which the particlesare suspended in the carrier gas and to envelop the fluidized particleswith vapor of silicon-containing precursor material, said fluidized bedbeing maintained at a nearly isothermal condition and at a temperatureabove the decomposition temperature of the silicon-containing precursor;c) agitating particles with agitating means in the fluidized bed whilesaid particles are suspended in the fluidized bed and the carrier gas;and d) passing an oxidizing gas into said fluidized bed separately fromsaid carrier gas containing vaporized tetramethyloxysilane and reactingsaid oxidizing gas with the vaporized tetramethyloxysilane on theparticles of the phosphor powder to form a continuous coating of silicaof predetermined thickness on the phosphor particles; e) annealing theparticles of the phosphor obtained from step (d); and f) vaporizing analuminum-containing precursor material selected from the groupconsisting of aluminum isopropoxide and tetramethylaluminum into aninert carrier gas to form a carrier gas containing vaporizedaluminum-containing precursor material; g) passing said carrier gascontaining vaporized aluminum precursor material through the phosphorpowder having a continuous coating of silica as produced by step (c) toform a fluidized bed in which particles are suspended in the carrier gascontaining vaporized aluminum precursor material and said fluidized bedbeing maintained at a nearly isothermal condition and at a temperatureabove the decomposition temperature of the aluminum-containingprecursor; and h) passing an oxidizing gas into said fluidized bed ofstep (f) separately from said carrier gas containing vaporizedaluminum-containing precursor material and reacting said oxidizing gaswith the vaporized aluminum-containing precursor material on theparticles of the phosphor powder having a continuous coating of silicato form a continuous coating of alumina of predetermined thickness onthe phosphor particles having a continuous coating of silica.
 10. Themethod according to claim 9 wherein the phosphor comprises a manganeseactivated zinc silicate phosphor.
 11. The method according to claim 9wherein the phosphor comprises a calcium halophosphate phosphor.
 12. Amethod for forming a continuous bi-layer coating in phosphor particlesin a phosphor powder comprising:a) a silicon containing precursormaterial selected from the group consisting of vaporizingtetramethyloxysilane and tetraethyloxysilane into an inert carrier gasto form a carrier gas containing vaporized silicon-containing precursormaterial; b) passing said carrier gas containing silicon-containingprecursor material through a mixture of phosphor powder and up to 1percent fluidizing aid to form a fluidized bed in which the particlesare suspended in the carrier gas and to envelop the fluidized particleswith vapor of silicon-containing precursor material, said fluidized bedbeing maintained at a nearly isothermal condition and at a temperatureabove the decomposition temperature of the silicon-containing precursor;c) agitating particles with agitating means in the fluidized bed whilesaid particles are suspended in the fluidized bed and the carrier gas;d) passing an oxidizing gas into said fluidized bed separately from saidcarrier gas containing vaporized tetramethyloxysilane and reacting saidoxidizing gas with the vaporized tetramethyloxysilane on the particlesof the phosphor powder to form a continuous coating of silica ofpredetermining thickness on the phosphor particles; e) vaporizing analuminum containing precursor material selected from the groupconsisting of aluminum isopropoxide and trimethylaluminum into an insertcarrier gas to form a carrier gas containing vaporized aluminumcontaining precursor material; f) passing said carrier gas containingvaporized aluminum precursor material through the phosphor powder havinga continuous coating of silica as produced by step (d) to form afluidized bed in which particles are suspended in the carrier gascontaining vaporized aluminum precursor material and to envelop thefluidized particles with vapor of aluminum containing precursor materialsaid fluidized bed being maintained at a nearly isothermal condition andat a temperature above the decomposition temperature of thealuminum-containing precursor; and g) passing an oxidizing gas into saidfluidized bed of step (f) separately from said carrier gas containingvaporized aluminum containing precursor material and reacting saidoxidizing gas with the vaporized aluminum containing precursor materialon the particles of the phosphor powder having a continuous coating ofsilica to form a continuous coating of alumina of predeterminedthickness of the phosphor particles having a continuous coating ofsilica.
 13. The method according to claim 12 further comprising:h)annealing the particles of phosphor obtained from step (g).
 14. Themethod according to claim 12 wherein the phosphor comprises a manganeseactivated zinc silicate phosphor.
 15. The method according to claim 12wherein the phosphor comprises a calcium halophosphate phosphor.